Okay, I've received the following reply from ASRock support:
The BIOS multiplier is already at 9.5, and the only thing related to CPU bus I can find is the FSB to CPU bus, which, when I set to 266MHz, still doesn't allow me to select the DRAM data rate at 1600.Please check under BIOS setup and verify your processor bus is 266MHz (x) P4=1333MHz with Ratio(multiplier)9.5
Check under SDRAM Frequency and voltage is set for 1.65 volts as you described your ram and bus 800MHz(x) 2 Dual Data Rate =1600MHz
ADATA replied with the following:
So, the answer to the bonus question seems to be "shoot low".Frequency: 1600mhz
Voltage: 1.55v to 1.65v (every board is different)
Is it time to go with a newer mobo with better DDR3 1600 support?